1. Field
The presently disclosed subject matter relates to a light scattering type particle detector.
2. Description of the Related Art
In a semiconductor manufacturing apparatus where the high integration and the fine structure have been enhanced, absorbed particles affecting the manufacturing yield have been reduced in size. Also, even in a medicine manufacturing apparatus or a food manufacturing apparatus, since bacteria are easily absorbed on particles floating in the air, a highly-purified environment has been required. In the semiconductor manufacturing apparatus, the medicine manufacturing apparatus and the food manufacturing apparatus, a highly-purified space, i. e., a clean room excluding fine particles as many as possible is required. Such a clean room is monitored by an easily-operated particle detector for detecting fine particles in real time. Particularly, ultra pure water requires an extremely high purifying degree where the number of fine particles whose diameter is less than 0.1 μm is not larger than 1/mL. Particle detectors of a light scattering type for detecting fine particles in the air or in water have been known, and a halogen lamp, a helium-neon (He—Ne) laser, a semiconductor laser diode (LD), an LD-excited solid state laser or the like is used as a light source for such particle detectors.
As illustrated in FIG. 25, which illustrates a first prior art light scattering type particle detector (see: FIG. 2 of Takashi MINKAMI, “Fine Particle Meter Using LD-Excited Solid-State Laser as Light Source”, Oct. 2001), this particle detector is a laser oscillator including a gas laser medium source such as He—Ne laser source 101, and mirrors 102 and 103 provided on both sides of the He—Ne laser source 101, wherein a particle detecting region 104 is provided. Since the strength of an electric field within the laser oscillator which depends upon the reflectivities of the mirrors 102 and 103 is as large as about 100 to 1000 times the strength of an electric field outside of the laser oscillator, the strength of scattered light SL scattered by fine particles passing through the particle detecting region 104 becomes very large. Therefore, such fine particles passing through the particle detecting region 104 at a flow rate of 300 mL/min, for example, can be detected by detecting the scattered light SL.
The above described first prior art particle detector, however, has the following problems:                (1) Since the He—Ne laser source 101 is formed by a glass tube, the He—Ne laser source 101 is small in mechanical strength, large in size, and short in life-time, thus increasing the size and manufacturing cost of the first prior art particle detector.        (2) In order to maintain the alignment stability of the laser oscillator, the laser oscillator has to be provided under a finely-temperature-controlled vibration removing environment, which would further increase the size and manufacturing cost of the first prior art particle detector.        (3) When air or water passes through the particle detecting region 104, turbulent flow and fluctuation of temperature would invite fluctuation of an optical path length of the laser oscillator due to the change of the refractive index of the laser oscillator, thus drastically changing the strength of oscillation of the laser oscillator to destabilize the detection of fine particles.        (4) The principle of light scattering is based upon Rayleigh scattering whose scattering cross section S is represented byS∝d6·λ4   (1)        
where “d” is a diameter of fine particles; and
λ is a wavelength of light of the laser oscillator.
Therefore, although fine particles with a diameter “d” of about 0.5 μm at most to increase their scattering cross section S can be detected, it is difficult to detect fine particles with a diameter of less than about 0.1 μm to decrease their scattering cross section S.
As illustrated in FIG. 26, which illustrates a second prior art light scattering type particle detector (see: FIG. 3 of Takashi MINAKAMI, “Fine Particle Meter Using LD-Excited Solid-State Laser as Light Source”, Oct. 2001, JP2002-151765A, JP2002-223019A and (US2004/001974A1), an laser-diode (LD)-excited solid-state laser oscillator is constructed by an LD device 201 as an exciting light source, a condensing lens 202, a solid-state laser such as a Nd3+:YVO4 laser crystal 203, and mirrors 204 and 205, wherein a particle detecting region 206 is provided. Since the strength of an electric field within the LD-excited laser oscillator which depends upon the reflectivities of the mirrors 204 and 205 is large about 100 to 1000 times as the strength of an electric field outside of the LD-excited laser oscillator, the strength of scattered light SL by fine particles passing through the particle detecting region 206 becomes very large. Therefore, such fine particles passing through the particle detecting region 206 at a flow rate of 300 mL/mm, for example, can be detected by detecting the scattered light SL.
The above described second prior art particle detector has the following advantage over the above-mentioned first prior art particle detector. That is, since the solid-state crystal 203 is large in mechanical strength, small in size, and long in life-time, thus decreasing the size and manufacturing cost of the second prior art particle detector.
The above-described second prior art particle detector, however, still has the following problems:                (1) In order to maintain the alignment stability of the LD-excited laser oscillator, the LD-excited laser oscillator has to be provided under a finely-temperature controlled vibration removing environment, which would still increase the size and manufacturing cost of the second prior art particle detector.        (2) When air or water passes through the particle detecting region 206, turbulent flow and fluctuation of temperature would invite fluctuation of an optical path length of the ID-excited laser oscillator due to the change of the refractive index of the LD-excited laser oscillator, thus drastically changing the strength of oscillation of the LD-excited Laser oscillator to destabilize, the detection of fine particles.        (3) The principle of light scattering is based upon Rayleigh scattering whose scattering cross section S is based upon the above mentioned formula (1). Therefore, it is still difficult to detect fine particles with a diameter of less than about 0.1 μm to decrease their scattering cross section S.        